The moment
Contact, then confirmation.
There is no button on the card and no charge in it. It does nothing until it is close to a phone, and then it does one thing.
So the phone has to carry the feedback. A ring that pulses while it is looking, a single settled state when it finds the card, a check when the write completed. No progress percentages, no spinner theatre.
The failure states matter more than the success. Held too far away, moved too early, wrong end of the phone: each of those gets a plain sentence telling you what to do differently.
Rules
Four rules for the interaction.
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I
Say where
Tell people to hold the top edge of the card to the top of the phone, once, plainly.
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II
Show looking
A soft pulse while searching. Motion means the phone is awake, not that work is being done.
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III
Confirm once
One clear state on success. No celebration.
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IV
Never half-write
A write either completed or it did not, and the screen must say which.
One second of contact.
No ceremony.